Mobile communications system refers generally to any telecommunications system which enables a wireless communication when users are moving within the service area of the system. A typical mobile communications system is a Public Land Mobile Network (PLMN). Often the mobile communications network is an access network providing a user with a wireless access to external networks, hosts, or services offered by specific service providers.
Third generation (3G) mobile systems, such as Universal Mobile Communications system (UMTS) and Future Public Land Mobile Telecommunications system (FPLMTS) later renamed as IMT-2000 (International Mobile Telecommunication 2000), are being developed. The 3G architecture comprises a radio access network RAN and a backbone network CN, which together provide a bearer network. The RAN provides the radio interface and the physical radio resources for user equipments UE. The RAN may be based on the UMTS terrestrial radio access network (UTRAN) or the EDGE, for example. The RAN is connected to one or more backbone or core networks CN which provide various telecommunications services, such as data, speech and messaging services. The CN may be a circuit switched (CS) domain network, such as a GSM (Global System for Mobile communication) based network, or a packet switched (PS) domain network, such as GPRS. The PS backbone network will provide the UE with Internet Protocol (IP) connectivity services called PS connectivity services herein. Thus, the UE can establish an IP connection to any IP host, IP network or IP service via the 3G access network.
One core or backbone network candidate is the general packet radio service GPRS which was initially developed for the GSM system. The GPRS infrastructure comprises support nodes such as a GPRS gateway support node (GGSN) and a GPRS serving support node (SGSN). As illustrated in FIG. 1, a SGSN1 is connected to the RAN so that the SGSN1 can provide a packet service for mobile user equipments. The intermediate RAN provides a radio access and a packet-switched data transmission between the 3G-SGSN and user equipment UE (mobile station). The GPRS backbone network is in turn connected to an external data network, e.g. to a public switched data network PSPDN, via GPRS gateway support nodes GGSN. The GPRS service thus allows packet switched data transmission to be provided between mobile data terminals and external data networks when the UTRAN (or the GSM) network functions as a radio access network RAN.
The main functions of the SGSN are to detect new GPRS mobile stations in its service area, handle the process of registering the new UEs along with the GPRS registers, send/receive data packets to/from the UE, and keep a record of the location of the UEs inside its service area. The subscription information is stored in a GPRS register (HSS, Home Subscriber Server). In order to access the GPRS services, a UE shall first make its presence known to the network by performing a GPRS attach. This operation establishes a logical link between the UE and the SGSN. More particularly, when the MS attaches to the GPRS network, i.e. in a GPRS attach procedure, the SGSN creates a mobility management context (MM context), and a logical link LLC (Logical Link Control) is established between the UE and the SGSN in a protocol layer. MM contexts are stored in the SGSN and UE. The MM context of the SGSN may contain subscriber data, such as the subscriber identity and location and routing information, etc.
The main functions of the GGSN nodes involve interaction with the external data networks. The GGSN may also be connected directly to a private corporate network or a host. The GGSN includes PDP addresses and routing information, i.e. SGSN addresses for active GPRS subscribers. The GGSN updates the location directory using routing information supplied by the SGSNs. The GGSN uses the routing information for tunnelling the protocol data units PDU from external networks to the current location of the UE, i.e. to the serving SGSN. The tunneling means that the data packet is encapsulated into another data packet during transfer from one end of the tunnel to another. The GGSN also decapsulates data packets received from UEs and forwards them to the appropriate data network. In order to send and receive GPRS data, the UE shall activate the packet data address that it wants to use, by requesting a PDP activation procedure. This operation makes the UE known in the corresponding GGSN, and interworking with external data networks can commence. More particularly, one or more PDP context is created in the MS and the GGSN and the SGSN, and stored in the serving SGSN in connection with the MM context. The PDP context defines different data transmission parameters, such as the PDP type (e.g. X.25 or IP), PDP address (e.g. IP address), quality of service QoS and NSAPI (Network Service Access Point Identifier). Two associated PDP contexts in different GSN nodes define a GTP tunnel. The tunnel is identified with a Tunnel ID (TID) which consists of an MM Context ID and the NSAPI. The UE activates the PDU context with a specific message, Activate PDP Context Request, in which it gives information on the PDP type, PDP address, required QoS and NSAPI, and optionally the access point name APN. The SGSN sends a create PDP context message to the GGSN which creates the PDP context and sends it to the SGSN. The SGSN sends the PDP context further to the UE in an Activate PDP Context Response message, and a virtual connection or link between the UE and the GGSN is established. The PDP context is stored in the UE, the SGSN and the GGSN. As a result the SGSN tunnels all the data packets from the UE to the GGSN, and the GGSN tunnels to the SGSN all data packets received from the external network and addressed to the UE.
ETSI 3GPP (European Telecommunications Standards Institute, 3rd Generation Partnership Project) specifications include IP based voice communications in release 2000 (in so called all-IP network), 3G TR 23.821 V1.0.1. In such an all-IP network it will be possible to perform also voice communication in IP network (voice over IP, VolP). However, also for VolP, call control signaling are specified, such as SIP and H.323. So, there is a call control signaling used for controlling the connection between the communication parties, as is also the case in the circuit switched voice communication. The element that performs the call control function in the IP network environment is typically called a call processing server. In addition, there are some additional requirements for the voice packet delivery because of the real time nature of the voice communication. Protocols such as RTP (Real Time transport Protocol) and QoS mechanisms are needed to handle that.
TETRA (Terrestrial Trunked Radio) is a standard defined by ETSI (European Telecommunications Standards Institute) for digital professional mobile radio or private mobile radio (PMR) systems. The TETRA system is developed primarily for professional and governmental users, such as the police, military forces, oil plants, etc. Group communications with a push-to-talk feature is one of the essential features of the TETRA network. It is characterized by extreme QoS requirements. Generally, in group voice communication with a “push-to-talk, release-to-listen” feature a group call is based on the use of a pressel (tangent) in a telephone as a switch: by pressing a pressel the user indicates his desire to speak, and the user equipment sends a service request to the network. The network either rejects the request or allocates the requested resources on the basis of predetermined criteria, such availability of resources, priority of the requesting user, etc. At the same time connection is also established to all other active users in the specific subscriber group. After the voice connection has been established, the requesting user can talk and the other users listen on the channel. When the user releases the pressel, the user equipment signals a release message to the network, and the resources are released. Thus, the resources are reserved only for the actual speech transaction or speech item.
The group communication implemented independently in IP-based 3G mobile communications networks would be an interesting feature. In the future, the all-IP network may be an option also in the TETRA. As noted above, voice communication development in the main stream 3G side is going towards the VolP. The VolP would be the most obvious candidate also for the group communication feature in the 3G mobile systems. The call control of the group communication is also likely to be based on the VolP signaling procedures and call processing server implementations. The general guidelines of ETSI standardization for the UE to network protocols in the all-IP 3G systems require that the protocols shall, as far as possible, conform the IETF “Internet standards”, such as SIP signaling, in order to achieve access independence and to maintain a smooth co-operation with wireline terminals across the Internet. However, an implementation which is based on the VolP signaling may not meet the QoS requirements of the push-to-talk group communication. Especially the need for quick connection establishment is considered to be a challenge in the IP network environment. Further, it would be advantageous to be able to implement the group communication feature in the 3G all-IP network independently of the IP voice communication method selected and the call processing server implementation and signaling procedures specified for it.